Roll-to-roll glass material attributes and fingerprint

ABSTRACT

A high-silica glass sheet has an average thickness of less than 150 microns and an average surface roughness over one or both of its two major surfaces of less than 1 nm. The glass sheet is formed using a roll-to-roll glass soot deposition and sintering process. The glass sheet may comprise a plurality of substantially parallel surface protrusions, which are visible only when a major surface of the glass sheet is viewed at an angle sufficiently removed from normal incidence.

BACKGROUND AND SUMMARY

The present invention relates generally to glass sheets and morespecifically to sintered glass sheets such as silica glass sheets madeusing a glass soot deposition and sintering process.

Glass sheet materials can be formed using a variety of differentmethods. In a float glass process, for example, a sheet of solid glassis made by floating molten glass on a bed of molten metal. This processcan be used to form glass sheets having uniform thickness and very flatsurfaces. However, float glass processes necessarily involve directcontact between the glass melt and the molten metal, which can lead toundesired contamination at the interface and less than pristine surfacequality. In order to produce high quality float glass sheets withpristine surface properties on both major surfaces, float glass istypically subjected to surface polishing steps, which add additionalexpense. Moreover, it is believed that the float process has not beenused to make ultra-thin, rollable glass sheets.

An additional method for forming glass sheet materials is the fusiondraw process. In this process, molten glass is fed into a trough calledan “isopipe,” which is overfilled until the molten glass flows evenlyover both sides. The molten glass then rejoins, or fuses, at the bottomof the trough where it is drawn to form a continuous sheet of flatglass. Because both major surfaces of the glass sheet do not directlycontact any support material during the forming process, high surfacequality in both major surfaces can be achieved.

Due to the dynamic nature of the fusion draw process, the number ofglass compositions suitable for fusion draw processing is limited tothose that possess the requisite properties in the molten phase (e.g.,liquidus viscosity, strain point, etc.). Further, although relativelythin glass sheets can be made via fusion draw, the process cannot beused to form ultra-thin, rollable high-silica glass sheets. Finally, theapparatus used in the fusion draw process can be expensive.

In addition to their limitations with respect to ultra-thin glass sheetmaterials, both float and fusion draw processes are largely impracticalsheet-forming methods for high-silica glass sheets due to the highsoftening point (˜1600° C.) of silica. Rather, silica glass substratesare typically produced by cutting, grinding and polishing silica ingotsproduced in batch flame-hydrolysis furnaces. Such a batch approach isextremely expensive and wasteful. Indeed, the requisite slicing andpolishing that would be needed to produce uniform, thin, flexible silicaglass sheets via flame-hydrolysis render the process prohibitivelyexpensive. Using known methods, Applicants believe that it is notcurrently feasible to form and polish both sides of a high-silica glasssheet having a thickness of less than 150 microns.

In view of the foregoing, economical, uniform, ultra-thin, flexible,rollable high-silica glass sheets having a high surface quality arehighly desirable. The high-silica glass sheets can comprise one or morelayers, components, or phases. Such glass sheets can be used, forexample, as photo mask substrates, LCD image mask substrates, and thelike.

A high-silica glass sheet has two major opposing surfaces having anaverage thickness of 150 microns or less and an average surfaceroughness over at least one of the two major surfaces of 1 nm or less.In an embodiment, an average surface roughness over both of the twomajor surfaces is 1 nm or less. Example high-silica glass sheets measureat least 2.5×2.5 cm². For example, a width of the glass sheet can rangefrom about 2.5 cm to 2 m and a length of the glass sheet can range fromabout 2.5 cm to 10 m or more. Indeed, the length of the glass sheet islimited in principle only by the deposition time, and can extend beyond10 m to 10 km or more. The glass sheet is formed using a roll-to-rollglass soot deposition and sintering process. In a further embodiment,the glass sheet comprises a plurality of pseudo-visible striations. Thestriations are caused by local thickness variations and are visible onlywhen a major surface of the glass sheet is viewed at an anglesufficiently removed from normal incidence.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus for formingultra-thin glass sheets;

FIG. 2 is a surface line scan of a sintered glass sheet according to oneembodiment; and

FIG. 3 is a surface line scan of a sintered glass sheet according to afurther embodiment.

DETAILED DESCRIPTION

A high-silica glass sheet has an average thickness of less than 150microns and an average surface roughness over one or both of its twomajor surfaces of less than 1 nm. The lateral dimensions of such a glasssheet can range from about 2.5 cm to 2 m wide and from about 2.5 cm to 3m long or longer. An apparatus for forming ultra-thin high-silica glasssheets is shown schematically in FIG. 1. The apparatus 100 comprises asoot-providing device 110, a soot-receiving device 120, a sootsheet-guiding device 130, and a soot sheet-sintering device 140. Amethod for forming a glass sheet is disclosed herein with reference tothe apparatus of FIG. 1.

Glass soot particles formed by the soot-providing device 110 aredeposited on a deposition surface 122 of the soot-receiving device 120.Advantageously, the soot-receiving device 120 is in the form of arotatable drum or belt and thus can comprise a continuous depositionsurface 122. The deposited soot particles 150 form a soot layer 152 onthe deposition surface 122. The soot layer 152, once formed, can bereleased from the deposition surface 122 as a free-standing, continuoussoot sheet 154. In preferred embodiments, the act of releasing the sootlayer 152 from the deposition surface 122 occurs without physicalintervention and can occur, for example, due to thermal mismatch, amismatch in coefficients of thermal expansion between the soot layer andthe deposition surface and/or under the effect of the force of gravity.After the soot sheet 154 is released from the soot-receiving device 120,a soot sheet-guiding device 130 can guide movement of the soot sheet 154through a soot sheet-sintering device 140, which sinters andconsolidates the soot sheet 154 to form an ultra-thin glass sheet 156.

A process of forming an ultra-thin glass sheet comprises providing aplurality of glass soot particles, depositing the glass soot particleson a deposition surface of a soot-receiving device to form a soot layer,releasing the soot layer from the soot-receiving surface to form a sootsheet, and sintering the soot sheet to form a glass sheet. Additionalaspects of the process and apparatus are disclosed in detail hereinbelow.

Although a variety of devices may be used to form glass soot particles,by way of example, the soot providing device 110 may comprise one ormore flame hydrolysis burners, such as those used in outside vapordeposition OVD, vapor axial deposition (VAD) and planar depositionprocesses. Suitable burner configurations are disclosed in U.S. Pat.Nos. 6,606,883, 5,922,100, 6,837,076, 6,743,011 and 6,736,633, thecontents of which are incorporated herein by reference in theirentirety.

The soot-providing device 110 may comprise a single burner or multipleburners. An example burner has an output surface having length l andwidth w. The output surface comprises N columns of gas orifices where Ncan range from 1 to 20 or more. In an embodiment, each orifice comprisesa 0.076 cm diameter hole. The length l of the output surface can rangefrom about 2.5 to 30.5 cm or more, and the width can range from 0. 1 to7.5 cm. Optionally, multiple burners can be configured into a burnerarray that can produce a substantially continuous stream of sootparticles over the width of the array.

A burner array, for example, may comprise a plurality of individualburners (e.g., placed end-to-end) configured to form and deposit atemporally and spatially uniform layer of glass soot. Thus, thesoot-providing device can be used to form a layer of soot having asubstantially homogeneous chemical composition and a substantiallyuniform thickness. By “uniform composition” and “uniform thickness” ismeant that the composition and thickness variation over a given area isless than or equal to 20% of an average composition or thickness. Incertain embodiments, one or both of the compositional and thicknessvariation of the soot sheet can be less than or equal to 10% of theirrespective average values over the soot sheet.

An example burner comprises 9 columns of gas orifices. During use,according to one embodiment, the centerline column (e.g., column 5)provides a silica gas precursor/carrier gas mixture. The immediatelyadjacent columns (e.g., columns 4 and 6) provide oxygen gas forstoichiometry control of the silica gas precursor. The next two columnson either side of the centerline (e.g., columns 2, 3, 7 and 8) provideadditional oxygen, the flow rate of which can be used to controlstoichiometry and soot density, and provide an oxidizer for the ignitionflame. The outermost columns of orifices (e.g., columns 1 and 9) canprovide an ignition flame mixture of, for example, CH₄/O₂ or H₂/O₂.Example gas flow rate ranges for a 9 column linear burner are disclosedin Table 1.

TABLE 1 Example gas flow rates for 9 column linear burner Gas Burnercolumn(s) Example flow rate OMCTS 5 15 g/min N₂ 5 40 SLPM O₂ 4, 6 18SLPM O₂ 2, 3, 7, 8 36 SLPM CH₄ 1, 9 36 SLPM O₂ 1, 9 30 SLPM

The soot-providing device may be held stationery during formation anddeposition of the soot particles or, alternatively, the soot-providingdevice may be moved (e.g., oscillated) with respect to the depositionsurface. A distance from the burner output surface to the depositionsurface can range from about 20 mm to 100 mm (e.g., 20, 25, 30, 35, 40,45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 mm).

Operation of the soot-providing device typically involves chemicalreactions between precursor chemicals (e.g., gaseous compounds) to formglass soot particles. Optionally, the chemical reactions can be furtherassisted by supplemental energy sources such as plasma or a supplementalheating device.

Silicon-containing precursor compounds, for example, can be used to formsoot sheets comprising silica soot particles that can be sintered toform silica glass sheets. An example silicon-containing precursorcompound is octamethylcyclotetrasiloxane (OMCTS). OMCTS can beintroduced into a burner or burner array together with H₂, O₂, CH₄ orother fuels where it is oxidized and hydrolyzed to produce silica sootparticles. Although the process of forming a glass sheet typicallycomprises forming a high-silica glass sheet, the process and apparatuscan be used to form other glass sheet materials as well.

As-produced or as-deposited, the soot particles may consist essentiallyof a single phase (e.g., a single oxide) such as in the example ofun-doped, high-purity silica glass. Alternatively, the soot particlesmay comprise two or more components or two or more phases, such as inthe example of doped silica glass. For instance, multiphase high-silicaglass sheets can be made by incorporating a titanium oxide precursor ora phosphorous oxide precursor into the OMCTS gas flow. Example titaniumand phosphorous oxide precursors include various soluble metal salts andmetal alkoxides such as halides of phosphorous and titanium (IV)isopropoxide.

In the example of a flame hydrolysis burner, doping can take place insitu during the flame hydrolysis process by introducing dopantprecursors into the flame. In a further example, such as in the case ofa plasma-heated soot sprayer, soot particles sprayed from the sprayercan be pre-doped or, alternatively, the sprayed soot particles can besubjected to a dopant-containing plasma atmosphere such that the sootparticles are doped in the plasma. In a still further example, dopantscan be incorporated into a soot sheet prior to or during sintering ofthe soot sheet. Example dopants include elements from Groups IA, IB,IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB and the rare earth series of thePeriodic Table of Elements.

The soot particles can have an essentially homogeneous composition, sizeand/or shape. Alternatively, one or more of the composition, size andshape of the soot particles can vary. For example, soot particles of amain glass component can be provided by a first soot-providing device,while soot particles of a dopant composition can be provided by a secondsoot-providing device. In certain embodiments, soot particles can mixand/or adhere to one another during the acts of forming and depositingthe soot particles to form composite particles. It is also possible thatthe soot particles are substantially prevented from adhering to eachother to form mixed particles prior to or while being deposited on thedeposition surface.

Referring still to FIG. 1, deposition surface 122 comprises a peripheralportion of the soot-receiving device 120 and can be formed of arefractory material. In an embodiment, the deposition surface 122 isformed of a material that is chemically and thermally compatible withboth the soot particles 150 and the deposited soot layer 152, and fromwhich the soot layer can be easily removed. Example soot-receivingdevices 120 comprise a coating or cladding of a refractory material(e.g., silica, silicon carbide, graphite, zirconia, etc.) formed over acore material of, for example, steel, aluminum or metal alloy. Furthersoot-receiving devices can comprise a unitary part consistingessentially of a suitable refractory material such as quartz.

The soot-receiving device 120 and particularly the deposition surface122 can be configured in a variety of different ways and have a varietyof shapes and/or dimensions. For example, a width of the depositionsurface can range from about 2 cm to 2 m, although smaller and largerdimensions are possible. A cross-sectional shape of the soot-receivingdevice 120 can be circular, oval, elliptical, triangular, square,hexagonal, etc., and a corresponding cross-sectional dimension (e.g.,diameter or length) of the soot-receiving device 120 can also vary. Forexample, a diameter of a soot-receiving device having a circular crosssection can range from about 2 cm to 50 cm. An example soot-receivingdevice 120 comprises a quartz cylinder having a 250 mm inner diameter, a260 mm outer diameter, and a 24 cm wide deposition surface.

In the examples of circular or oval cross-sections, the depositionsurface 122 can comprise a closed, continuous surface, while in theexamples of elliptical, triangular, square or hexagonal cross-sections,the deposition surface can comprise a segmented surface. Byappropriately selecting the size and dimensions of the soot-receivingdevice 120, a continuous or semi-continuous soot sheet can be formed.

The deposition surface 122 can include regular or irregular patterningin the form of raised or lowered protrusions across a range of lengthscales. The patterning can range from one or more discrete facets to ageneral roughing of the surface. Thus, a deposited soot layer canconform to the patterning in the deposition surface. The pattern formedin the soot surface can be retained in the deposited surface of the sootsheet as it is separated from the deposition surface and, in turn,preserved in the sintered surface of the resulting glass sheet resultingin an embossed glass sheet. In a variation of the above-describeddeposition surface-derived embossing, one or both of the depositedsurface and the free surface of a soot sheet can be patterned after itis removed from the deposition surface but prior to sintering. Forexample, Applicants have patterned a soot sheet surface with afingerprint. Upon sintering of the soot sheet, the fingerprint patternis retained in the resulting glass sheet.

In certain embodiments, the soot-receiving device 120 is rotated duringthe act of depositing soot particles 150 in order to form a soot layer152 thereon. The rotation can be unidirectional, e.g., clockwise orcounter-clockwise. A direction of rotation according to one embodimentis indicated by arrow A in FIG. 1. Optionally, the soot-receiving devicemay oscillate during the soot deposition process, i.e., the rotationdirection may change intermittently. A linear velocity of the depositionsurface 122 of the soot-receiving device 120 can range from 0.1 mm/secto 10 mm/sec (e.g., 0.1, 0.2, 0.5, 1, 2, 3, 4, 5 or 10 mm/sec). Inscale-up, it is believed that the linear velocity of the depositionsurface can be increased up to 1 m/sec or higher.

Soot particles 150 are deposited on only a portion of the depositionsurface 122, and the deposited soot layer 152 is removed to form afree-standing continuous or semi-continuous soot sheet 154 having lengthL. As illustrated in FIG. 1, a width of the deposited layer 152 (andnominally of the soot sheet 154) is W.

In certain embodiments, the soot sheet can be continuously formed on andcontinuously removed from the deposition surface. During formation of asoot layer, soot particles bond to a certain degree with each other andwith the deposition surface. The higher the average temperature of thesoot particles when they are being deposited, the more likely they areto bond with each other and form a dense and mechanically robust sootsheet. However, higher deposition temperatures also promote bondingbetween the soot particles and the deposition surface, which caninterfere with releasing of the soot sheet. To obtain a substantiallyuniform temperature across the deposition surface, the soot-receivingdevice can be heated or cooled either from the inside, the outside, orboth.

Bonding between soot particle and the deposition surface can becontrolled by controlling a temperature gradient between a locationwhere the soot particles are deposited and a location where the sootlayer is released to form a soot sheet. For instance, if the soot layerand the deposition surface have sufficiently different coefficients ofthermal expansion (CTEs), the release may occur spontaneously due tostress caused by the temperature gradient. In certain embodiments,removal of the deposited soot layer from the deposition surface can bemade easier by forming a soot layer having a width W that is less thanthe width of the deposition surface 122.

During the act of separating the soot layer from the deposition surface,a direction of motion of the separated soot sheet can be substantiallytangential to a release point on the deposition surface. By“substantially tangential” is meant that the direction of motion of thesoot sheet relative to a release point on the deposition surfacedeviates by less than about 10 degrees (e.g., less than 10, 5, 2 or 1degrees) from a direction that is tangential to the deposition surfaceat the release point. Maintaining a substantially tangential releaseangle can reduce the stress exerted on the soot sheet at the releasepoint.

For a soot-receiving device having a circular or oval cross section, thecurvature of the deposition surface is a function of the cross-sectionaldiameter(s) of the soot-receiving device. As the diameter increases, theradius of curvature increases, and stresses in the deposited sootdecrease as the shape of the deposited soot sheet approaches that of aflat, planar sheet.

In embodiments, the soot sheet has sufficient mechanical integrity tosupport its own mass (i.e., during the acts of removal from thedeposition surface, handling and sintering) without fracturing. Processvariable that can affect the physical and mechanical properties of thesoot sheet include, inter alia, the thickness and density of the sootsheet, the curvature of the deposition surface, and the temperature ofthe soot sheet during formation.

The soot sheet comprises two major surfaces, only one of which contactsthe deposition surface during formation of the soot layer. Thus, the twomajor surfaces of both the soot sheet and the sintered glass sheetderived therefrom may be characterized and distinguished as the“deposited surface” and the opposing “free surface.”

In an example of a soot sheet comprising at least 90 mole % silica, anaverage soot density of the soot sheet can range from about 0.3 to 1.5g/cm³, e.g., from about 0.4 to 0.7 g/cm³, or from about 0.8 to 1.25g/cm³, and an average thickness of the soot sheet can range from 10 to600 μm, e.g., 20 to 200 μm, 50 to 100 μm or 300 to 500 μm.

In certain embodiments, particularly those involving continuous sootsheet and/or sintered glass sheet production, continuous movement of thesoot sheet away from the deposition surface after its release can beaided by a soot sheet guiding device 310. The soot sheet guiding device130 can directly contact at least a portion of the soot sheet 154 inorder to aid movement and provide mechanical support for the soot sheet.

To maintain a high surface quality of the soot sheet, the soot sheetguiding device 130 may contact only a portion (e.g., an edge portion) ofthe soot sheet 154. In certain embodiments, the soot sheet guidingdevice comprises a pair of clamping rollers that can grip an edgeportion of the soot sheet and guide the soot sheet through a soot sheetsintering device.

Using a soot sheet guiding device, a continuous soot sheet can be fedinto a sintering/annealing zone of a soot sheet sintering device 140where at least a portion of the soot sheet is heated at a temperatureand for a period of time sufficient to convert the heated portion intodensified glass. For example, a soot sheet of high purity silica can besintered at a temperature ranging from about 1000° C. to 1900° C., e.g.,from about 1400° C. to 1600° C. to form an ultra-thin silica glass sheet156. The sintering temperature and the sintering time can be controlledin order to form a sintered glass sheet that is essentially free ofvoids and gas bubbles.

As used herein, sintering refers to a process whereby glass sootparticles are heated below their melting point (solid state sintering)until they adhere to each other. Annealing is a process of cooling glassto relieve internal stresses after it was formed. Sintering andannealing can be carried out sequentially using the same or differentapparatus.

The glass sheet formation process may be controlled in order to minimizestrain (e.g., sagging) of both the soot sheet and the resulting glasssheet. One way to minimize strain is to orient the soot sheetsubstantially vertically during sintering. According to embodiments, anangle of orientation of the soot sheet with respect to a verticalorientation can be less than 15 degrees (e.g., less than 10 or 5degrees). An additional way to minimize strain is to apply a tensilestress to the soot sheet and/or the glass sheet during the act ofsintering. A tensile stress can be applied in a plane of a major surfaceusing, for example, clamping rollers or clamping conveyor belts. Tensilestress can be applied to a soot sheet in an amount effective to tensionthe soot sheet substantially flat.

In addition to minimizing gravity-induced strain, the application of atensile stress can have noticeable effects on the sintering process.During sintering, the density of the soot sheet increases as a glasssheet is formed. In order to conserve mass, the density increase isaccompanied by an overall decrease in volume. In the absence of atensile stress, substantial lateral shrinkage of the soot sheet wouldoccur. By applying a tensile stress to the soot sheet, however, thevolume change is realized mainly through a change in thickness.Moreover, by applying a tensile stress to the soot sheet duringsintering, sintering can be performed on non-vertically oriented sootsheets without risk of substantial deformation.

A variety of different soot sheet-sintering devices, including resistiveheating and induction heating devices, can be used to sinter the sootsheet. The thermal history of both the soot sheet and the glass sheetcan affect the final thickness, composition, compositional homogeneityand other chemical and physical properties of the final product. A glasssheet can be formed by applying heat to one or both of the major surfaceof the soot sheet. During sintering, various parameters can becontrolled including temperature and temperature profile, time, andatmosphere.

Though a sintering temperature can be selected by skilled artisan basedon, for example, the composition of the soot sheet to be sintered, asintering temperature can range from about 1000° C. to 1900° C. Further,a homogeneous temperature profile, which is achievable with bothresistive and induction heating sources, can be used to createhomogeneity within the final glass sheet. By “homogeneous temperatureprofile” is meant a sintering temperature that varies by less than 20%(e.g., less than 10 or 5%) over a predetermined sample area or samplevolume.

In embodiments where an edge portion of the soot sheet is held andguided by the soot sheet-guiding device, that edge portion is typicallynot sintered by the sintering device. Rather, the soot sheet-sinteringdevice will sinter only a center-portion of the soot sheet. For example,in one embodiment, the center 10 cm of a soot sheet having an averagethickness of about 400 microns and a total width of 24 cm was heated toproduce a sintered glass sheet having a width of about 10 cm and anaverage thickness of about 100 microns. Prior to sintering, an averagedensity of the soot sheet is about 0.5 g/cm³. By not sintering an edgeportion of the soot sheet in embodiment where the edge portion is heldand guided by a soot sheet-guiding device, between about 5 and 95% ofthe total soot sheet area can be sintered.

In addition to controlling the temperature and the temperature profileduring sintering, the gas ambient surrounding the soot sheet/glass sheetcan also be controlled. Specifically, both the total pressure as well asthe partial pressure of suitable sintering gases can be selected inorder to control the sintering process. In certain embodiments, acontrolled gas mixture can comprise one or more active or inert gasessuch as, for example, He, O₂, CO, N₂, Ar or mixtures thereof

During the act of sintering, the soot sheet maybe held stationery withina sintering zone or moved continuously or semi-continuously through sucha zone. For example, in a continuous glass sheet forming process, a rateof production of the soot sheet as it is released from the sootdeposition surface may be substantially equal to a rate of translationof the soot sheet through the sintering zone. Sintering may be performedvia one or more passes through a sintering zone using the same ordifferent sintering conditions. A distance from the heater to the sootsurface can range from about 1 mm to 10 mm (e.g., 1, 2, 3, 4, 5, 6, 7,8, 9 or 10 mm).

Once formed, the glass sheet may be divided into discrete pieces by asuitable cutting device. For example, a laser can be used to cut theglass sheet into smaller pieces. Further, before or after cutting, thesintered glass can be subjected to one or more post-sintering processes,such as edge removal, coating, polishing, etc. A long ribbon of sinteredglass sheet can be reeled by a reeling device into a roll. Optionally,spacing materials such as paper sheet, cloth, coating materials, etc.can be inserted in between adjacent glass surfaces in the roll to avoiddirect contact there between.

The process and apparatus disclosed herein are suited for making sootsheets and sintered glass sheets comprising a high percentage of silica,e.g., “high-silica” glass sheets. By “high-silica” is meant a glasscomposition comprising at least 50 mole % silica glass, e.g., greaterthan 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 98, 99, 99.5 or 99.9 mole %silica.

Flexible sintered glass sheets, including long glass ribbons, can beformed. Sintered glass sheets such as high-silica glass sheets can havean average thickness of 150 microns or less (e.g., less than 150, 100,50, or 25 microns). Example glass sheets have a thickness of 10, 30, 50or 100 μm. By controlling width of the deposited soot layer, the widthof the sintering zone, and the amount of deposition time, it is possibleto independently control both the width and the length of sintered glasssheets. A length of the glass sheet can range from about 2.5 cm to 10km. A width of the glass sheet can range from about 2.5 cm to 2 m, whichcan represent from about 9 to 95% of a width of the soot sheet.

The process can be used to form high surface quality glass sheets (e.g.,glass sheets having low surface waviness, low surface roughness, andwhich are essentially free from scratches). The above-disclosed process,which can include an initial step of forming a soot sheet on a roll, anda final step of reeling a sintered, flexible glass sheet onto a roll,can be referred to as a “roll-to-roll” process. The resulting glasssheets, including high-silica glass sheets, can be characterized by anumber of properties including composition, thickness, surfaceroughness, surface uniformity and flatness.

Roughness and other surface features may be measured using contact ornon-contact methods. Contact methods involve dragging a measurementstylus across a sample surface, and can comprise instruments such asprofilometers. Exemplary non-contact methods include interferometry,confocal microscopy, electrical capacitance and electron microscopy.

Surface metrology data including surface roughness were measured using aZygo white light interferometric microscope (New View 5000, ZygoCorporation, Middlefield, Conn.). The surface roughness calculation isbased on a roughness profile that has been filtered from a raw profiledata and contains a calculated mean line.

The roughness profile contains n ordered, equally spaced points along aone-dimensional or two-dimensional trace, and yi is the verticaldistance from the mean line or plane to the ith data point. Height isassumed to be positive in the up direction, away from the bulk material.As used herein, Ra is the arithmetic average of absolute values asexpressed by Equation 1, and Rq is the root mean square (rms) roughnessas expressed by Equation 2.

$\begin{matrix}{R_{a} = {\frac{1}{n}{\sum\limits_{i = 1}^{n}{y_{i}}}}} & (1) \\{R_{q} = \sqrt{\frac{1}{n}{\sum\limits_{i = 1}^{n}y_{i}^{2}}}} & (2)\end{matrix}$

In one example glass sheet, average Ra and Rq values were 1.11 and 1.41nm, respectively, with corresponding standard deviations of 0.23 and0.28 nm over a sample area measuring 0.18×0.13 mm². In a further exampleglass sheet, average Ra and Rq values were 0.59 and 0.78 nm,respectively, over a sample area measuring 0.16×0.12 mm². These datawere measured on the deposited surface of the sintered glass sheet. Inembodiments, average surface roughness (Ra) over one or both majorsurfaces is less than 1 nm, though the average surface roughness canrange from about 1 to 5 nm (e.g., 1, 2, 3, 4 or 5 nm). Sample-to-samplevariability in the roughness data may be caused by handling or debris,particularly prior to sintering.

The uniformity of the glass sheets can be affected by the existence ofsubstantially parallel protrusions on one or both major surfaces of theglass sheet. The protrusions are formed parallel to a fabricationdirection of the soot sheet, and are an optical manifestation of a localdifference in physical thickness. The protrusions can be observedoptically only when viewing the glass sheet at angle with respect to themajor surface normal. Notably, when examining a glass sheet at normalincidence, the thickness variation is essentially invisible. Thus, atnormal incidence, what is seen is a homogeneous index of refraction.

The protrusions are characteristic of the soot sheet formation processand, without wishing to be bound by theory, are believed to be anartifact of the burner configuration. Because the protrusions arebelieved to be unique to a particular burner geometry, they may be usedas a process-identifying fingerprint.

Profile data were taken using a Surfcom 200OSD (Carl Zeiss) contour androughness profiling system. FIGS. 2 and 3 are surface line scans ofexample sintered glass sheets. The surface line scans were measured onthe deposited surface of the glass sheets. FIG. 2 shows across-sectional scan of a surface protrusion having an overall height ofabout 40 microns and a width at the base of about 9 mm. FIG. 3 shows across-sectional scan of adjacent surface protrusions having overallheights of between about 3 to 5 microns and a width at the base rangingfrom about 1 to 2 mm.

According to embodiments, the protrusions may be characterized by anoverall height ranging from about 1 to 100 microns (e.g., 1, 2, 5, 10,20, 50 or 100 microns), and a width at the base ranging from about 0.5to 10 mm (e.g., 0.5, 1, 2, 5 or 10 mm).

As used herein, “soot layer” or “layer of soot” refers to a stratum ofessentially homogeneously-distributed glass particles that areoptionally bonded with each other. The layer generally has an averagetotal thickness that is greater than or equal to an average diameter ofindividual particles. Further, a soot layer may comprise a single sootlayer having an essentially homogeneous composition or multiple sootlayers each having an essentially homogeneous composition.

In embodiments where the soot layer comprises multiple layers, onespecies of glass particles can form a first soot layer, while a secondspecies of glass particles can form a second soot layer adjacent to thefirst soot layer. Thus, respective soot layers can have distinctivecompositional and/or other properties. Moreover, in an interfacialregion between the first and second layers, blending of the two speciesof particles can occur such that the composition and/or properties atthe interface of contiguous layers may deviate from the bulk valuesassociated with each respective layer.

Reference herein to a “glass sheet” includes both sheet materialscomprising a plurality of glass soot particles (i.e., soot sheets) andsheet materials made of sintered glass. As is typically understood inthe art, a sheet has two major opposing surfaces that are typicallysubstantially parallel to each other, each having an area larger thanthat of other surfaces. A distance between the two major surfaces at acertain locations is the thickness of the sheet at that particularlocation. A sheet may have a substantially uniform thickness between themajor surfaces, or the thickness can vary spatially either uniformly ornon-uniformly. In certain other embodiments, the two major surfaces canbe non-parallel, and one or both of the major surfaces can be planar orcurved.

As used herein, “sintered glass” refers to a glass material having adensity of at least 95% of a theoretical density (Dmax) for a glassmaterial having the same chemical composition and microstructure underconditions of standard temperature and pressure (STP) (273 K and 101.325kPa). In certain embodiments, it is desired that the sintered glass hasa density of at least 98%, 99% or 99.9% of Dmax under STP.

Additional aspects of ultra-thin glass sheet formation using a glasssoot deposition and sintering process are disclosed in commonly-ownedU.S. application Ser. No. 11/800,585, filed May 7, 2007, the contents ofwhich are incorporated herein by reference in their entirety.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “metal” includes examples having two or moresuch “metals” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It is also noted that recitations herein refer to a component of thepresent invention being “programmed” or “configured” in a particularway. In this respect, such a component is “programmed” or “configured”to embody a particular property, or function in a particular manner,where such recitations are structural recitations as opposed torecitations of intended use. More specifically, the references herein tothe manner in which a component is “programmed” or “configured” denotesan existing physical condition of the component and, as such, is to betaken as a definite recitation of the structural characteristics of thecomponent.

EXAMPLES

The invention will be further clarified by the following examples.

Example 1 Fabrication of a Single-Layer Soot Sheet

A soot layer comprising 99+ mole % silica was produced using five linearburners. The five 2.5 cm wide burners were mounted adjacently in aburner array to produce a uniform, 12.5 cm wide soot stream. Each burnerincluded 9 parallel rows of gas orifices. The gas orifices each measure0.075 cm in diameter.

A precursor gas mixture comprising OMCTS (5 g/min) entrained in N₂ (20SLPM) was flowed through the centerline row of orifices. Oxygen gas wasflowed through three adjacent rows of orifices on each side of thecenterline row. The oxygen gas flow through the immediately adjacentrows was 5 SLPM, while the oxygen gas flow through the next pair of rowswas 20 SLPM. The gas flow through the final (outside) orifice rowscomprised a mixture of CH₄ (12 SLPM) and O₂ (10 SLPM).

The burners were positioned approximately 10 cm from the depositionsurface of a cylindrical soot-receiving device. The soot-receivingdevice had a diameter of about 38 cm and a wall thickness of about 0.64cm. The soot-receiving device was rotated to provide a linear surfacespeed of 1 mm/sec.

Soot from the burner array was directed to the deposition surface and alayer of soot approximately 200 microns thick and 15 cm wide wasdeposited. An average density of the center 13 cm of the soot sheet wasapproximately 1.1 g/cm³. The 13 cm wide soot sheet created at the higherdensity was released from the drum, augmented by a stream of airsupplied by an airknife. The airknife supplied approximately 20 SLPM ofair through a 25.4 cm wide airknife body, which was directed at thedeposition surface. The soot sheet was manually grabbed by theperipheral edges and directed to a wind drum. The wind drum wasapproximately 41 cm in diameter. Five meters of soot sheet were woundonto the drum.

Example 2 Sintering of a Single-Layer Soot Sheet

A soot sheet as described above in Example 1 but having a thickness ofabout 400 microns was fabricated. The thickness was increased bydecreasing the rotation speed of the soot-receiving device andincreasing the OMCTS flow rate.

A soot sheet measuring approximately 1.2 m long and 7.6 cm wide wassintered by first gripping peripheral edges of the soot sheet betweenrollers that maintained contact along the length of the sample within asintering zone, and then passing the soot sheet through a heat sourcethat heated the soot sheet to approximately 1500° C. to form adensified, clear, sintered glass. The sintered glass had a finalthickness of approximately 100 microns.

The sintered sheet having un-sintered peripheral edges was removed fromthe gripping rollers and the un-sintered edges were trimmed using alaser that was traversed at approximately 3 mm/s along the length of thesheet.

Example 3 Effect of Process Conditions on Soot Sheet Thickness andDensity

A designed experiment was conducted in which the (i) rotational velocityof the soot-receiving device, (ii) distance from the burner to thedeposition surface, and (iii) 02 flow rate in the outermost burnercolumns (i.e., columns 1 and 9) were systemically varied. The effects ofthese variables on the thickness and density of the resulting soot sheetwere measured and the data is shown in Table 2. Using the selectedparameters, as seen in the tabulated data, a thickness of the soot sheetvaries from about 250 to 700 microns, and a bulk density of the sootsheet varies from about 0.35 to 0.7 g/cm³. Generally, the soot sheetthickness and density increase with decreasing velocity of thesoot-receiving device.

TABLE 2 Effect of process conditions on soot sheet thickness and bulkdensity Burner-Deposition Soot Bulk Soot Run Velocity Surface O₂Thickness Density # [mm/sec] Distance [mm] [slpm] [μm] [g/cm³] 1 3 27.926 350 0.37 2 2 27.9 30 500 0.40 3 2 27.9 22 420 0.47 4 1 27.9 26 6100.62 5 2 32.3 26 370 0.52 6 1 32.3 22 690 0.65 7 1 36.7 26 660 0.62 8 232.3 26 400 0.55 9 2 32.3 26 370 0.53 10 3 32.3 22 240 0.50 11 2 36.7 22420 0.42 12 1 32.3 30 640 0.68 13 2 36.7 26 360 0.49 14 3 36.7 26 3000.40 15 3 32.3 30 290 0.44 16 2 36.7 30 390 0.45 17 2 32.3 26 400 0.51

During sintering/annealing to form a glass sheet, the soot sheetthickness typically decreases (and the density increases) by a factor ofabout 4, e.g., by a factor of from about 3 to 5.

Example 4 Multi-Component Glass Sheets

Multi-component glass sheets having binary (SiO₂/TiO₂) and ternary(SiO₂/TiO₂/P₂O₅) compositions were made by introducing titanium andoptionally phosphorus precursors into the OMCTS flow. Compositions ofthe resulting multi-component high-silica glass sheets, expressed inpercent by weight of the respective components, are disclosed in Table3. In the Table 3 data, the silica content was determined by gravimetrywhile the titanium and phosphorus content(s) were determined byinductively coupled plasma/direct coupled plasma (ICP/DCP). As a resultof using different measurement techniques, the compositions of therespective components may not total 100 wt. %.

TABLE 3 Compositions of multi-component high-silica glass sheets RunSiO₂ [wt. %] TiO₂ [wt. %} P₂O₅ [wt. %] 993 94.1 4.46 0.1 994 93.4 5.02 0

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Sincemodifications combinations, sub-combinations and variations of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and their equivalents.

1. A high-silica glass sheet having two major opposing surfaces andcomprising: at least 50 mole % silica; an average thickness between thetwo major opposing surfaces of about 150 microns or less, and furtherincluding at least one attribute selected from the group consisting of:(i) an average surface roughness over at least one of the two majorsurfaces of about 1 nm or less; (ii) a plurality of substantiallyparallel surface protrusions formed on at least one of the two majoropposing surfaces; and (iii) a length of at least 2.5 cm and a width ofat least 2.5 cm.
 2. The high-silica glass sheet according to claim 1,wherein the glass sheet comprises at least 80 mole % silica.
 3. Thehigh-silica glass sheet according to claim 1, wherein the glass sheetcomprises at least 95 mole % silica.
 4. The high-silica glass sheetaccording to claim 1, wherein the glass sheet comprises at least onedopant selected from the group of elements consisting of Groups IA, IB,IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB and the rare earth series of thePeriodic Table of Elements.
 5. The high-silica glass sheet according toclaim 1, wherein the glass sheet has an average thickness of less thanabout 50 microns.
 6. The high-silica glass sheet according to claim 1,wherein the glass sheet has an average surface roughness over both majorsurfaces of about 1 nm or less.
 7. The high-silica glass sheet accordingto claim 1, wherein an average height of the protrusions ranges fromabout 1 to 100 microns.
 8. The high-silica glass sheet according toclaim 1, wherein the protrusions on a major surface are visible onlywhen the glass sheet is viewed at greater than a critical angle fromnormal incidence with respect to the major surface.
 9. The high-silicaglass sheet according to claim 8, wherein the critical angle is greaterthan 5 degrees.
 10. The high-silica glass sheet according to claim 1,wherein a plurality of substantially parallel protrusions are formed ineach of the two major opposing surfaces.
 11. The high-silica glass sheetaccording to claim 1, wherein the glass sheet has a width of at leastabout 7.5 cm and a length of at least about 30 cm.
 12. The high-silicaglass sheet according to claim 1, wherein one or both of a depositedsurface and a free surface of the glass sheet comprise embossing.
 13. Aroll of glass sheet material comprising the high-silica glass sheetaccording to claim
 1. 14. A high-silica glass sheet having two majoropposing surfaces and comprising: at least 50 mole % silica; an averagethickness between the two major opposing surfaces of about 150 micronsor less; and an average surface roughness over at least one of the twomajor surfaces of about 1 nm or less.
 15. A high-silica glass sheethaving two major opposing surfaces and comprising: at least 50 mole %silica; an average thickness between the two major opposing surfaces ofabout 150 microns or less; and a length of at least 2.5 cm and a widthof at least 2.5 cm.